Device for detecting the boundary between different brightness regions of an object

ABSTRACT

A device for automatically determining the position of a boundary between light and dark regions of an object such as an IC mask pattern or the like, the device including an optical system for forming a point or line symmetrical double image, a light beam modulator means for providing a position signal from an image formed through said optical system, a photoelectric converter means for converting a light signal modulated by the light beam modulator means into an electrical signal, and a phase detector means for converting the electrical signal from the photoelectric converter into a DC signal, whereby the position of the boundary may be determined in a highly accurate and stable manner irrespective of any relatively great or small brightness and difference between the light and dark regions of the object, and without any adverse effect resulting from such variable factors as the variation in brightness of the image, the aging of the detector element and operator errors.

United States Patent 119 Iisuka 11 June 19, 1973 l DEVICE FOR DETECTINGTHE BOUNDARY BETWEEN DIFFERENT BRIGHTNESS REGIONS OF AN OBJECT [75]Inventor: Kiyoshi lisuka, Kawasaki-shi, Japan [73] Assignee: NipponKogaku K.K., Tokyo, Japan [22] Filed: June 23, 1971 [21] Appl. No.:155,839

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney- Raymond J. McElhannon, Lorimer P. Brooks,.loseph M.Fitzpatrick et al.

[57] ABSTRACT A device for automatically determining the position of aboundary between light and dark regions of an object such as an IC maskpattern or the like, the device including an optical system for forminga point or line symmetrical double image, a light beam modulator meansfor providing a position signal from an image formed through saidoptical system, a photoelectric converter means for converting a lightsignal modulated by the light beam modulator means into an electricalsignal, and a phase detector means for converting the electrical signalfrom the photoelectric converter into a DC signal, whereby the positionof the boundary may be determined in a highly accurate and stable mannerirrespective of any relatively great or small brightness and differencebetween the light and dark regions of the object, and without anyadverse effect resulting from such variable factors as the variation inbrightness of the image, the aging of the detector element and operatorerrors.

9 Claims, 14 Drawing Figures ACAMP PHASE INDICATING (DETECTOR METERREFERENCE OSCILLATOR Patented June 19, 1973 3,740,152

2 Sheets-Sheet l I PRIOR ART PHOTO RESPONSIVE ELEMENT 2 DIFFERENTIAL 3cIRcuIT REFERENCE PHOTO REsPONsIvE ELEMENT PRIOR ART lg- FIG. 3

PHOTOELECTRIC -16 |7- CONVERTER A.c.AIvIR 30 I5 I3 I I8 I 20 I4E%-VIBRATOR PHASE INDICATING ['DETEOTOR METER (1 REFERENCE OSCILLATORPatented June 19, 1973 2 Sheets-Sheet 2 VIBRATOR FIG.6(0)

F lG.6(b)

1 DEVICE FOR DETECTING THE BOUNDARY BETWEEN DIFFERENT BRIGHTNESS REGIONSor AN OBJECT BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a device for determining the position of a boundaryof a brightness such as the boundary between regions of differentbrightnesses or the outline of a projected image provided by an outlineprojector or the like.

2. Description of the Prior Art A device for automatically measuring theoutline-of a projected image provided by a projector or the like isknown. Such a known device includes a measuring photoresponsive elementand a reference photoresponsive element which is approximately half thesize of the measuring element. A differential circuit is connectedwith'the two photoresponsive elements for detecting the outputdifference between these elements. An optical image comprising a lightregion and a dark region contiguous to each other along a boundary iscaused to approach the photoresponsive elements. When the boundary inthe optical imagereaches a certain position with respect to themeasuring photoresponsive element, the outputs of the two elementsbecome equal to each other so that the differential circuit provides azero output. Such a position is invariable even if the brightness of thelight region in the optical image is reduced to a certain extent.However, if the brightness of the dark regionis higher than a certainlevel, the mea- FIGS. 2a and 2b show curves representing quantities ofdistributed light in the device of FIG. 1 for illustrating the operationthereof.

FIG. 3 is a block diagram of an example of the device according to thepresent invention; FIGS. 4a to 40, 5a to Sc and 6a to 6c are'illustrations of the operation of the device of FIG. 3, wherein FIG. '4illustrates the relationship between an image and a scanning slit, FIG.5 illustrates the variations in quantity of distributed light resultingfrom the scanning, and FIG. 6 illustrates the waveforms of electricalsignals provided by a photoelectric converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 7 reference is first had toFIGS. 1 and 2 which relate to suring element will always receive agreater quantity of light than the reference element and this in turnwill result in a .variation in the output of the differential circuit,which makes it impossible to determine the position of .a boundary bythe output 'of such circuit.

' SUMMARY OF THEINVENTIQN boundary between light and dark regions of anobject with high accuracy and high stability irrespective of anyrelatively great or small difference in brightness between the light anddark regions, and without any adverse effect resulting from thevariation in the brightness of the image and/or from the aging ofdetectorelement and without any error occurring from individual measurerto measurer. j

The device according to the present invention comprises an opticalsystem for forming a point or line symmetrical double image, a lightbeam modulator means for providing a position signal from an imageformed through the optical system, a photoelectric converter means forconverting a light signal modulated by the light beam modulator meansinto an electrical signal, and a phase detector means for converting theelectrical signal from the photoelectric converter means into a DCsignal. v

An embodiment of the present invention will be described hereunder ingreater detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram oftheboundary determining device according to the prior art.

the conventional boundary determining device. As shown in FIG. 1, thedevice of the prior art includes a measuring photoresponsive element 1and a reference photoresponsive element 2. The former element isapproximately twice as large as the latter. The twophotoresponsiveelements 1 and 2 are connected with a differential circuit 3 whichdetects the output difference between these elements. An optical image 4to be. measured comprises a light region 5 and a dark region 6 which arecontiguous to each other along a boundary 7. The quantity of lightdistributed in the optical image 4, wherein the light in the dark regionis almost equal to zero, is shown by a curve 9 in FIG. 2(a), where line8 designates zero quantity of light.. The optical image is moved alongthe photoresponsive elements from left to right as viewed in FIG. 1.When the image reaches a position where its boundary 7 is aligned withthe center line of the measuring photoresponsive element 1 as indicatedby a reference line A, the output of the measuring photoresponsiveelement 1 becomes equal to the output of the reference photoresponsiveelement 2, because the light from'region 5 impinges only on thereference element2 and on the right-hand half of element 1, so that theoutputof the differential circuit 3 becomes zero. In this position, theright-hand half of the measuring element 1 is equal in dimensions to thereference element 2, and therefore such output is invariable even if thebrightness of the light region 5 is reduced to provide a quantity ofdistributed light as indicated by a curve 10 in FIG; 2(a). However, whenthe difference in brightness between the light region 5 and the darkregion 6 is reduced, as for example due to a relatively large lightquantity in region 6 as indicated by a curve 11 in FIG. 2(b),themeasuring photoresponsive element 1 will always receive a greaterquantity of light than the reference photoresponsive element 2 due tothe continuous reception of light by theleft-hand half thereof. Insuchcase, therefore, it becomes impossible to determine the boundary betweenthe light and dark regions of the image by the use of the output of thedifferential circuit 3. I

Referring now to FIG. 3, there is shown an embodiment of the presentinvention. An object to be measured is designated by numeral 4 andcomprises a light region 5 and a dark region 6 which are contiguous toeach other along a boundary 7. Numeral l2 designates a known opticalsystem for forming a point or line symmetrical double image. A scanningslit plate 13 is disposed adjacent to the optical system 12 and formedwith a slit 14, which extends perpendicularly to the scanning directionof the slit plate and is parallel to the boundary 7 in the object to bemeasured. A vibrator 15 is provided for vibrating the slit plate 13. Theslit plate 13 with the slit 14 and the vibrator 15 together constitute alight beam modulating means 30. A photoelectric converter 16 fordetecting a light beam passed through the slit 14 is electricallyconnected with an AC amplifier 17 for amplifying a signal from thephotoelectric converter 16. The amplifier 17 in turn is connected with aphase detector 18. A reference oscillator 19 is connected, on the onehand, with the vibrator l and on the other hand, with thephase detector18, which in turn is connected with an indicating meter 20.

With the described arrangement, the object 4 such as an IC mask patternor an image projected from a projector onto a screen is focused in thescanning slit 14 through the optical system 12 for forming a point orline symmetrical double image. The scanning slit 14 is vibrated by thevibrator in synchronism with a signal of a predetermined frequency fromthe reference oscillator 19 so as to scan the image formed through theoptical system 12. The reference oscillator 19 may be omitted when acommercially available predetermined frequency is used. A light beampassed through the scanning slit l4 impinges on the photoelectricconverter 16, which converts such light beam into an electrical signal.The electrical signal thus provided and the positional relationshipbetween the object 4 and the measuring device will now be described byreference to FIGS. 4 to 6.

FIG. 4 shows the relationship between the image of the'object 4 formedthrough the optical system 12 and the center position of the scanningamplitude of the scanning slit 14. FIG. 5 illustrates the variation inquantity of distributed light resulting from the scanning of the lightbeam passed through the scanning slit 14. FIG. 6 illustrates thewaveforms of the electrical signals provided by the photoelectricconverter 16. In FIG. 4, (a) refers to the case where the boundary 7 inthe object 4 is slightly deviated in one direction from the optic axisof the optical-system 12, (b) refers to the case where the boundary 7 isaligned with the optic axis, and (c) refers to the case where theboundary 7 is slightly deviated in the other direction from the opticaxis. As seen in FIG. 4(a), the characteristic of the optical system 12results in the formation of image portions 21 and 21 where the images ofthe light and dark regions 5 and 6 of the object 4 overlap each other,image portion 22 where the image of the light region 5 appears in theform of a double image, and images 25 and 25' representing the boundary7 in the object 4. The area designated by numeral 23 shows the casewhere the scanning slit 14 is positioned at the center of the scanningamplitude. In FIG. 4 (c), numeral 24 designates an image portion wherethe image of the dark region 6 of the object is caused to appear in theform of a double image by the optical system 12. Sequentiallyvariedrelative position of the object and the device causes the image of theobject 4 formed through the optical system 12 to be varied in the orderof FIG. 4(a), (b) and (c) or vice versa. The variations in the quantityof light passed through the scanning slit.14 during one scanning cycleof the slit 14 in the cases of FIG. 4(a), (b) and (c) are indicated bycurves 27, 28 and 29 in FIG. 5(a), (b) and (0), respectively. In FIG.5(a), lines 26 and 26 indicate the variations in the quantity of lightprovided when it is assumed that scanning is effected only on one of twosymmetrical image components ofa point or line Y symmetrical image.Actually, two images are superposed one upon the other, and thereforethe actual variation in the quantity of light is represented by thecurve 27 which is the sum of the curves 26 and 26. This is also the casewith (b) and (c). In the case of (b), it is seen that no variationoccurs in the quantity of light passed through the scanning slit 14.

As clearly seen in FIG. 5, the photoelectric converter 16 providessignals having the waveforms as shown in FIGS. 6(a), (b) and (0). FIG.6(a), (b) and (c) each shown only the variable component of thesesignals.

The waveforms shown in FIGS. 6(a), (b) and (c) indicate that thevariation in the signal from the photoelectric converter 16 becomes zerowhen the boundary 7 in the object 4 is completely aligned with the opticaxis of the optical system 12 and that when the boundary 7 is deviatedfrom the optic axis to left or right as viewed in FIG. 3, there isprovided an AC signal whose phase has been reversedin accordance withthe direction of such deviation. The signal from the photoelectricconverter 16 is amplified by the AC amplifier l7 and then applied to thephase detector 18. If the AC amplifier 17 is a band-pass amplifier whosecenter frequency is equal to the oscillation frequency of the referenceoscillator 19, it may remove any noise component to thereby enabledetection to be effected with higher sensitivity and higher stability.As is well known, the phase detector may be a phase sensitive rectifierand may provide a signal of positive or negative polarity in accordancewith the phase of the input signal with respect to the reference signal,and it provides a signal whose magnitude is proportionate to theamplitude of the input signal. The output of the phase detector becomeszero for zero input and for even harmonics of the reference signal.Thus, when the boundary 7 in the object 4 is aligned with the optic axisof the optical system 12, the output of the phase detector 18 becomeszero so that the position of the boundary 7 can be determined. If thevariation in the brightness of the object in the vicinity of theboundary 7 is irregular, the line 28 'in FIG. 5 will not berectilinear/Therefore, the variation in the signal from thephotoelectric converter 16 does not become zero as shown in FIG. 6(b)but provides an AC signal. However, such AC signal is even harmonics ofthe reference signal as will be readily appreciatedfrom the fact thatthere is provided a point symmetrical image, and therefore the outputsignal of the phase detector 18 becomes zero as in the case describedabove, whereby no erroneous detection occurs. The output of the phasedetector 18 is a DC signal which enables the position of the boundary 7to be determined by the pointer of the indicating meter 19.

Provision of an additional circuit which uses the output signal of thephase detector 18 to generate a pulse at a point where the output passeszero would obviously make the device versatile for various usagesincluding automatic measurement.

Although the present invention has been shown and described as embodiedin a light beam modulating device for obtaining positional informationby causing the scanning slit to vibrate while causing it to scan a lightbeam passed therethrough, it will be apparent to those skilled in theart that the slit may be stationary while the light beam may be vibratedby a vibrating mirror. For example, as illustrated in FIG. 7, the slitplate 13 may be disposed in a fixed relation with respect to thephotoelectric converter 16 and the lens 12 so that light from the image4 which passes through the lens 12 is reflected by a movable mirror 50through the slit 14 to the converter 16. Thus, in the modification ofthe embodiment of the invention illustrated in FIG. 7, the vibrator 15is arranged to vibrate the mirror 50, thereby producing the requiredscanning action.-

Thus, according to the present invention, the position of the boundarybetween the light and dark regions which must be determined for themeasurement of 1C patterns or the like can be determined with highaccuracy but without any adverse effect arising from errors of theindividual measurers. Moreover, even an object having a low contrast oflight and shade can be measured stably irrespective of such variablefactors as the brightness of the illuminating light source and thesensitivity of the detecting photoelectric converter.

1 claim:

1. A device for determining the position of a boundary between differentbrightness regions of an object, comprising:

an optical system for forming a symmetrical double image of an object;

light beam modulator means for producing a light signal from said imageformed through said optical system;

photoelectric converter means for converting said light signal modulatedby said light beam modulator means into an electrical signal; and

phase detector means coupled to said modulator means and said convertermeans for converting said electrical signal from said photoelectricconverter means into a DC signal.

2. A device as defined in claim 1, wherein said light beam modulatormeans comprises a scanning slit plate formed with a scanning slit forpassing therethrough a light beam from said object, and a vibrator forvibrating said slit plate.

3. A device as defined in claim 2, wherein said scanning slit platemoves reciprocally in one direction and said scanning slit extendsperpendicularly to the scanning direction of said slit plate andparallel to the boundary of said object.

4. A device as defined in claim 2, wherein said modulator means furthercomprises a reference oscillator, and wherein said scanning slit plateis vibrated by said vibrator in synchronism with a signal of apredetermined frequency applied to said vibrator by said referenceoscillator.

5. A device as defined in claim 2, wherein said modulator means furthercomprises means for applying an electrical signal of predeterminedfrequency to said vibrator, and wherein said scanning slit plate isvibrated by said vibrator in synchronism with said predeterminedfrequency.

6. A device as defined in claim 1, wherein said light beam modulatormeans comprises a scanning slit plate formed with a scanning slit and avibrating mirror for vibrating a light beam passed through said scanningslit.

7. A device as defined in claim ll, further comprising an AC amplifierfor amplifying the electrical signal from said photoelectric convertermeans, and an indicating meter for indicating the DC signal from saidphase detector means representing the position of said boundary.

8. A device for determining the position of a boundary between differentbrightness regions of an object, comprising:

an optical system for forming a symmetrical double image of said object;means for generating a signal of a predetermined frequency;

scanning means connected to said signal generating means for scanningsaid image formed through said optical system in synchronism with saidsignal from said signal generating means to thereby provide a positionalsignal representing the position of the boundary;

photoelectric converter means for converting said positional signal intoan electric signal; and

phase detector means coupled to said photoelectric converter means andto said signal generating means for using said signal from said signalgenerating means as a reference signal to convert said electrical signalfrom said photoelectric converter means into a DC. signal representingthe position of said boundary.

9. A device for determining the relative positions of a reference lineand a boundary between different brightness regions of an object,comprising:

an optical system for forming a symmetrical double image of an object;

light beam position modulator means for producing a light signal fromsaid image, wherein said light signal is position modulated with respectto said reference line;

photoelectric converter means for converting said light signal into anelectrical signal; and

phase detector means coupled to said modulator means and said convertermeans for providing an output signal which is proportional to theposition of said boundary relative to said reference line.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,470,152 September 30 1969 Ottmar Zipp et a1 It is certified that errorappears in the above identified patent and the t said Letters Patent arehereby corrected as shmm below:

Column 8 second formula cancel "-Br".

Signed and sealed this 26th day of May 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. JR.

Attcsting Officer Commissioner of Patents

1. A device for determining the position of a boundary between differentbrightness regions of an object, comprising: an optical system forforming a symmetrical double image of an object; light beam modulatormeans for producing a light signal from said image formed through saidoptical system; photoelectric converter means for converting said lightsignal modulated by said light beam modulator means into an electricalsignal; and phase detector means coupled to said modulator means andsaid converter means for converting said electrical signal from saidphotoelectric converter means into a DC signal.
 2. A device as definedin claim 1, wherein said light beam modulator means comprises a scanningslit plate formed with a scanning slit for passing therethrough a lightbeam from said object, and a vibrator for vibrating said slit plate. 3.A device as defined in claim 2, wherein said scanning slit plate movesreciprocally in one direction and said scanning slit extendsperpendicularly to the scanning direction of said slit plate andparallel to the boundary of said object.
 4. A device as defined in claim2, wherein said modulator means further comprises a referenceoscillator, and wherein said scanning slit plate is vibrated by saidvibrator in synchronism with a signal of a predetermined frequencyapplied to said vibrator by said reference oscillator.
 5. A device asdefined in claim 2, wherein said modulator means further comprises meansfor applying an electrical signal of predetermined frequency to saidvibrator, and wherein said scanning slit plate is vibrated by saidvibrator in synchronism with said predetermined frequency.
 6. A deviceas defined in claim 1, wherein said light beam modulator means comprisesa scanning slit plate formed with a scanning slit and a vibrating mirrorfor vibrating a light beam passed through said scanning slit.
 7. Adevice as defined in claim 1, further comprising an AC amplifier foramplifying the electrical signal from said photoelectric convertermeans, and an indicating meter for indicating the DC signal from saidphase detector means representing the position of said boundary.
 8. Adevice for determining the position of a boundary between differentbrightness regions of an object, comprising: an optical system forforming a symmetrical double image of said object; means for generatinga signal of a predetermined frequency; scanning means connected to saidsignal generating means for scanning said image formed through saidoptical system in synchronism with said signal from said signalgenerating means to thereby provide a positional signal representing theposition of the boundary; photoelectric converter means for convertingsaid positional signal into an electric signal; and phase detector meanscoupled to said photoelectric converter means and to said signalgenerating means for using said signal from said signal generating meansas a reference signal to convert said electrical signal from saidphotoelectric converter means into a D.C. signal representing theposition of said boundary.
 9. A device for determining the relativepositions of a reference line and a boundary between differentbrightness regions of an object, comprising: an optical system forforming a symmetrical double image of an object; light beam positionmodulator means for producing a light signal from said image, whereinsaid light signal is position modulated with respect to said referenceline; photoelectric converter means for converting said light signalinto an electrical signal; and phase detector means coupled to saidmodulator means and said converter means for providing an output signalwhich is proportiOnal to the position of said boundary relative to saidreference line.